The semiconductor wafers with which this invention deals are of the type from which a very large plurality of individual electronic micro circuits or components are made using well-known coating, masking and impurity introduction techniques. Such wafers are highly fragile and easily damaged, because of their thinness (of the order of 10-20 mils), large diameter (2 to 6 inches), highly polished faces and microscopic surface steps and grooves. These wafers can be rendered useless for device fabrication by any of many kinds of contamination, abrasion or damage, even minute scratches. Therefore, extraordinary care in handling is called for, and in particular it is highly desirable to support and transport wafers in a manner which minimizes hand and machine contact therewith, and any mechanical shock or abrasion.
Previous expedients for handling and transport of wafers have ranged from the very primitive to the overly complex, and have included simple platens or sleds on which wafers are laid horizontally and transported, for example by moving conveyor belts or chains, linear air bearings or tracks, clip supports of various designs, and cassettes. All of these expedients have necessitated a fairly large degree of handling and loading by technicians, and have been more or less difficult to make compatible with automated transports and means for loading and unloading at many of the necessary wafer processing stations particularly those involving the need to transport wafers into and from a vacuum environment. Elaborate load locks and loading apparatus have been necessitated, or further manual handling at such stations. Such disadvantages have been one key factor inhibiting extensive adoption of individual wafer processing on a continuous serial basis, as opposed to batch loading and processing.
Other disadvantages of these prior expedients have been at least as serious. Many have not held the wafers positively during movement, stops, or other shocks, with the result that the wafers undergo movement relative to other parts, giving rise to damage. Other expedients, while providing more positive support, have cut yields by engaging the wafers over some portion of their surface area, or failed to position the wafer consistently in a predetermined position, or become unreliable when faced with wafers whose diameters vary due to presently unavoidable normal manufacturing tolerances. Most support expedients have not permitted processing of both sides of the wafer, nor provided any positive protection against contact with other parts, nor tolerated processing equipment being applied to the wafer without risking its dislodgement from the transport means, nor readily accommodated to highly simplified automated loading.
Clips of the prior art as described in U.S. Pat. No. 4,306,731 to the same inventor, assigned in common with this patent, are used in very tight geometry, which results in close tolerances. Such clips are made of strip material, are difficult to manufacture and are highly stressed. Because the clips are highly stressed, the temperature to which these clips are exposed must be limited to prevent annealing the spring material. In addition, the wafers must be oriented so flats do not engage the clips, as clip deflection is too small to accommodate such a situation.